Colombia produces about 442 tonnes of cocaine annually, according to the United Nations.

Last year, authorities seized 252 tonnes of the narcotic.

Authorities say the Usuga Clan, which emerged after the mass demobilisation of right-wing paramilitaries a decade ago, ships tonnes of cocaine from Colombia to Central America and on to the US.

Seizures of that size are rare but not unprecedented on a global scale. In March 2007, the US Coast Guard seized nearly 20 tonnes of cocaine found on a cargo ship - the Gatun - headed from the Panama Canal to the Mexican port of Topolobampo.

In 2001, the Coast Guard found 13 tonnes of cocaine aboard a Belize-flagged fishing boat that aroused suspicion while off the Mexican coast because it had no working fishing equipment and few fish.